1. Field of the Invention
The present invention relates to suspensions for the solvent-free production of silicone resin-bonded papers based on sheet silicates.
2. Description of the Background
Silicone resin-bonded papers based on sheet silicates are important intermediates for the production of moldings which are used, for example, as insulators in the electrical industry, particularly where considerable heating occurs during the use of the electrical apparatus, for example in toasters, electric heaters, soldering apparatuses and hairdryers.
Solvent methods or suspension methods are known for the production of these silicone resin-bonded papers (A. Tomanek: Silicone & Technik, page 145; C. Hanser Verlag Munich 1990).
In the solvent method, a paper produced separately, for example, as described in DD 292 944, and comprising sheet silicates of the likes of mica, is impregnated with a solution of silicone resin and then dried. This silicone resin-bonded paper produced in this manner can then be processed as a prepreg in the desired manner. The particular disadvantage of this two-stage process is that, in the second stage, the paper is impregnated with a solution of a silicone resin in an organic, generally aromatic solvent and therefore the solvent vapors formed during the subsequent drying have to be handled by a technically complicated procedure. BE 758 263 describes the hydrophobization of the mica by means of silanes or silanols to increase the resistance of the mica to the solvents used. However, since these papers have insufficient mechanical strength for subsequent applications, they must then be further strengthened with solutions of resins, for example epoxy resins, in organic solvents.
One-stage processes which permit paper formation and binding of the paper in one step are likewise known (DE 11 26 467). In this process, paper formation is effected in the presence of a silicone resin solution. However, solvent-containing air is likewise formed during the drying of the papers, and in addition the wastewater is contaminated with solvent.
These ecological problems are substantially solved by the suspension method. This method is described, inter alia, in DE 44 26 213. Here, a pulp of solid silicone resin powder, sheet silicates (generally lamellar mica), water and optionally further additives is produced. This pulp is processed by suitable known methods to give papers which, after drying, can be used as prepregs for the production of mechanically stable sheets of resin-bonded sheet silicate material, so-called micanite sheets. The disadvantage of this process is that the electrical and mechanical characteristics (for example, the tensile strength, the flexural strength and the water absorption) of the micanite sheets thus produced or of the moldings or insulation materials produced therefrom are poorer than in the case of those micanite sheets which are produced from silicone resin-bonded papers manufactured by the solvent method. A further deficiency of the papers produced by known suspension methods is that a great deal of dust is generated during the processing of sheets to moldings, for example by punching, sawing and cutting. A need therefore continues to exist for silicone resin-bonded papers of improved mechanical and electrical properties.